Sexton, now 41, made 128 appearances for Canterbury, moved to Irish province Ulster when Solomons coached there, and is now rugby academy manager of the Tasman provincial team.
The appointment of a third New Zealand coach means that South African Super Rugby teams will be 60 per cent led by New Zealanders next season, unless the Lions are relegated.
Former All Blacks coach John Mitchell is currently the Lions coach and the Sharks are coached by John Plumtree, also a New Zealander.
Sexton will be assisted by Brad Mooar who is also a New Zealander and he leaves his current position as head coach of the Canterbury under-19s to become the Kings' specialist skills coach.
The Southern Kings are a politically motivated team formed to develop rugby talent in the Eastern Cape Region of South Africa.
The South African Rugby Union have guaranteed the Kings a place in the 2013 Super Rugby tournament but the Union have yet to reveal how this will be implemented as there are already 15 teams and expansion has been ruled out.